netlink: add ns_id as a generic argument to receive_callback

ns_id will be used to collect sockets and other per-netns
resources

travis-ci: success for Dump and restore nested network namespaces (rev4)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>